1. Field of the Invention
The present invention is in the field of formation of electronic components and more particularly relates to a novel method of forming on miniaturized electronic components conductive paths and land areas of varying thickness.
2. Prior Art
In the manufacture of electronic components such as I.C. devices, capacitors, and the like, and particularly miniaturized devices of the type described, it is highly desirable to utilize the surface area of the substrate in a most efficient manner so as to enable a maximum amount of circuitry to be concentrated in a minimal area. Devices of the type described necessarily incorporate a multiplicity of conductive paths leading between individual elements of the circuitry and also leading to termination areas for connection to ancillary components. Certain of the internal conductors may carry very limited amounts of current and thus, may be of very small cross section area. Others of the internal conductors on the same substrate may carry significant amounts of current and must be of substantially greater cross sectional area. In addition, where connections between conductor paths and terminals leading to the exterior of the device are to be effected, it is highly desirable that the interface between the terminals and conductive paths be of greater conductivity than the conductive path itself.
Conductive paths are typically formed of expensive materials such as silver and gold and it is accordingly highly desirable that the paths be tailored to the conductance capacities required so that the quantum of conductive material employed is neither too large nor too small.
In the manufacture of miniaturized electronic devices, conductive paths of varying current carrying capacities were fabricated in accordance with one or the other of two accepted methods. In accordance with a first method, the conductive material was screened onto the surface of the substrate through a mask which incorporated narrow pathways where the current carrying capacity of the conductor was not great and wider pathways where larger currents were to be encountered, such as at the power supply terminals to an I.C. device. In all instances in accordance with the described method, the thickness of the conductive path is regulated by the thickness of the mask and thus, greater conductivity could be achieved only by forming a wider path with the result that substantial surface areas of the substrate were covered where high conductivity paths were requried.
In order to more economically employ the surface area of the substrate, it has been suggested to form the conductive paths of varying thickness rather than of varying width. The methods heretofore employed for forming variable thickness paths have embodied drawbacks which have precluded their successful practice.
In accordance with a first method, one or more series of conductive paths is formed on the substrate using a mask of a first thickness resulting in the formation of conductive paths corresponding to the thickness of the mask. Thereafter, where it is desired to form conductive paths or areas of greater thickness the first mask is removed and a second mask of greater thickness substituted whereupon deposits of conductor forming material corresponding to the greater thickness of the second mask could be effected on the substrate. As will be readily recognized when it is considered that certain of the substrate areas may be only fifty thousands of an inch in each direction, the coordination of positioning of a first mask and a second mask present almost insuperable difficulties, particularly when it is considered that miniaturized parts must typically be formed hundreds or thousands at a time.
An attempt has been made to fabricate parts of varying thicknesses with a single masking step. Such a process is suggested by the Presco Screen Division of the AMI Corporation of North Branch, New Jersey. Briefly stated, the Presco process comprises providing a screen having a flat surface placed against the substrate to receive the conductive components. The upper surface of the screen is etched away in certain areas where it is intended that thin films are to be deposited. Through going apertures are formed through the screen both in the etched away areas and in the full thickness areas of the screen. In use, electroding inks or pastes are forced through the various apertures in the screen, namely the apertures in the etched away reduced thickness areas and in the full thickness areas. A soft, highly compliant squeegee is passed over the surface of the screen, the squeegee entering into the reduced thickness etched away areas so as to reduce the column or thickness of material extending through the apertures in the reduced thickness area to a level corresponding essentially to the height of the etched away area. The squeegee also removes excess material so that the apertures in the full thickness area are substantially filled. Following application of the squeegeeing step, the mask is removed from the substrate leaving increments of a first thickness in registry with the apertures formally in the full height area of the mask, and leaving thinner increments of the electrode material at those areas in registry with the etched away portions of the mask.
As will be apparent from the description of the Presco screen, the Presco process is of limited utility particularly where miniaturized components are to be processed. Firstly, since the formation of half height electroding increments in the etched area is dependent upon the squeegee being able to extend into the etched area while adjacent portions of the squeegee are supported on the full height area, it is obvious that the Presco device can be used only in connection with large and well separated patterns, since thinning of the electroded material is dependent upon the material of the squeegee extruding into the etched away upper surface of the Presco screen. As a practical matter, the Presco procedure can only be used in respect of features on the order of one hundred mils or more, thus rendering the Presco screening device unsuitable for processing miniaturized electronic devices. Finally, a further shortcoming of the Presco system resides in the fact that due to the extreme softness required of the squeegee for penetration into the etched areas, the squeegee will likewise partially project into the apertures in the full thickness portions of the Presco screen and necessarily displace increments of material from the full height paths. That is to say, since the pressure which must necessarily be applied to the Presco squeegee must be sufficient to project increments of the squeegee into the etched away areas, the pressure will also be sufficient to project portions of the squeegee into the full height apertures displacing portions of the conductive filler material therein.